Semiconductor devices that are fabricated using InP (Indium Phosphide) based substrates tend to outperform those fabricated using Gallium arsenide (GaAs) substrates, in that they have lower effective electron mass and a higher conduction band discontinuity (ΔEc). However, fabrication and processing of Indium-containing substrates, such as InP, is more difficult than the processing of GaAs substrates. In addition, they are significantly more costly. There is a need, therefore, for techniques to form or grow Indium-based semiconductors on GaAs substrates.
A major difficulty in fabricating a high Indium content semiconductor on a GaAs substrate is related to the differences in the corresponding crystal lattice constants. In general, the lattice constant of an Indium-based material is larger than that of GaAs. As such, an Indium-based semiconductor material formed directly on a GaAs substrate gives rise to a mismatch in lattice constants. Strain caused by this mismatch result in dislocations in the semiconductor material thereby degrading device performance and or reducing yield.
In the past, strain-relieving buffer layers have been used to compensate for lattice mismatch. U.S. Pat. No. 5,770,868, which is incorporated herein by reference in its entirety, discloses an Antimony (Sb) buffer layer that makes a lattice constant transition between a GaAs substrate and a high-Indium epitaxially deposited semiconductor. The buffer layer is an epitaxial layer including atoms of two Group III elements (e.g., Gallium and Aluminum), and atoms of two Group V elements (e.g., Arsenic and Antimony). The ratio of atoms of at least one group are varied along the depth of the buffer layer, in a manner which makes a transition of the lattice constant between that of the GaAs substrate and the high-Indium semiconductor material. However, when the Gallium and Indium content is changed, the resulting bandgap and the lattice constant are also changed. Thus, flexibility can be limited.
What is needed, therefore, are techniques for forming Indium containing semiconductor materials on a GaAs substrate that afford flexibility in adjusting the lattice constant and bandgap independently and the GaxIn1-xAsySb1-y semiconductor compounds.